X-ray sources have been used for over a century. One common x-ray source design is the reflection x-ray source 80, an example of which illustrated in FIG. 1. The source comprises a vacuum environment (typically 10−6 torr or better) commonly maintained by a sealed vacuum tube 20 or active pumping, and is manufactured with sealed electrical leads 21 and 22 that pass from the negative and positive terminals of a high voltage source 10 outside the vacuum tube 20 to the various elements inside. The source 80 will typically comprise mounts 30 which secure the vacuum tube 20 in a housing 50, and the housing 50 may additionally comprise shielding material, such as lead, to prevent x-rays from being radiated by the source 80 in unwanted directions.
Inside the tube 20, an emitter 11 connected through the lead 21 to the high voltage source 10 serves as a cathode and generates a beam of electrons 111. A target 100 supported by a target substrate 110 is electrically connected to the opposite high voltage lead 22 and target support 32, and therefore serves as an anode. The electrons 111 accelerate towards the target 100 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons 111 into the target 100 induces several effects, including the generation of x-rays 888, some of which exit the vacuum tube 20 through a window 40 or aperture.
In some prior art embodiments, the target 100 and substrate 110 may be integrated or comprise a solid block of the same material, such as copper (Cu). Electron optics (electrostatic or electromagnetic lenses) may also be provided to guide and shape the path of the electrons, forming a more concentrated, focused beam at the target. Likewise, electron sources comprising multiple emitters may be provided to provide a larger, distributed source of electrons.
When the electrons collide with a target 100, they can interact in several ways. These are illustrated in FIG. 2. The electrons in the electron beam 111 collide with the target 100 at its surface 102, and the electrons that pass through the surface transfer their energy into the target 100 in an interaction volume 200, generally defined by the incident electron beam footprint (area) times the electron penetration depth.
An equation commonly used to estimate the penetration depth for electrons into a material is Potts' Law [P. J. Potts, Electron Probe Microanalysis, Ch. 10 of A Handbook of Silicate Rock Analysis, Springer Netherlands, 1987, p. 336)], which states that the penetration depth x in microns is related to the 10% of the value of the electron energy E0 in keV raised to the 3/2 power, divided by the density of the material:
                              x          ⁡                      (            µm            )                          =                  0.1          ×                                    E              0              1.5                        ρ                                              [                  Eqn          .                                          ⁢          1                ]            For less dense materials, such as a diamond substrate, the penetration depth is much larger than for a material with greater density, such as most elements used for x-ray generation.
There are several energy transfer mechanisms that can occur. Throughout the interaction volume 200, electron energy may simply be converted into heat. Some absorbed energy may excite the generation of secondary electrons, typically detected from a region 221 located near the surface, while some electrons may also be backscattered, which, due to their higher energy, can be detected from a somewhat larger region 231.
Throughout the interaction volume 200, including in the regions 221 and 231 near the surface and extending approximately 3 times deeper into the target 100, x-rays 888 are generated and radiated outward in all directions. A typical x-ray spectrum for radiation from the collision of 100 keV electrons with a tungsten target is illustrated in FIG. 3. The broad spectrum x-ray radiation 388, commonly called “bremsstrahlung”, arises from electrons that were diverted from their initial trajectory. These continuum x-rays 388 are generated throughout the interaction volume, shown in FIG. 2 as the largest shaded portion 288 of the interaction volume 200. As was shown in FIG. 1, the x-ray source 80 will typically have a window 40, which may additionally comprise a filter, such as a sheet or layer of aluminum, that attenuates the low energy x-rays, producing the modified energy spectrum 488 shown in FIG. 3. Characteristic x-rays, shown in FIG. 3 and indicated by 988, are primarily generated in a fraction of the electron penetration depth, shown as the second largest shaded portion 248 of the interaction volume 200. The relative depth is influenced in part by the energy of the electrons 111, which typically falls off with increasing depth. The actual dimensions of this interaction volume 200 may vary, depending on the energy and angle of incidence of the electrons, the surface topography and other properties (including local charge density), and the density and atomic composition of the target material.
Although x-rays may be radiated isotropically, as was illustrated in FIG. 2, only the x-ray radiation 888 within a small solid angle produced in the direction of a window in the source will be useful. X-ray brightness (also called “brilliance” by some), defined as the number of x-ray photons per second per solid angle in mrad2 per area of the x-ray source in mm2, can be increased by adjusting the geometric factors to maximize the collected x-rays.
As illustrated in FIGS. 4A-4C, the surface of a target 100 in a reflection x-ray source is generally mounted at an angle θ (as was also shown in FIG. 1). X-ray radiation through a window 440 is shown for a set of five equally spaced radiation spots 408 for three take-off angles: θ=60° in FIG. 4A, θ=45° in FIG. 4B, and θ=30° in FIG. 4C. It can be seen that for lower take-off angles (e.g. FIG. 4C), the apparent spot size is reduced and thus apparent brightness increases.
In principle, it may appear that a take-off angle of θ=0° would have the largest possible brightness. In practice, radiation at 0° occurs parallel to the surface of a solid metal target for conventional sources, and since the x-rays must propagate along a long length of the target material before emerging, most of the produced x-rays will be attenuated (reabsorbed) by the target material, reducing brightness. Thus a source with take-off angle of around 6° to 15° (depending on the source configuration, target material, and electron energy) is conventionally used.
Another way to increase the brightness of the x-ray source is to use a target material with a higher atomic number Z, as efficiency of x-ray production for bremsstrahlung radiation scales with increasingly higher Z. Furthermore, the x-ray radiating material should ideally have good thermal properties, such as a high melting point and high thermal conductivity, in order to allow higher electron power loading on the source to increase x-ray production. Table I lists several materials that are commonly used for x-ray targets, several additional potential target materials (notably useful for specific characteristic lines of interest), and some materials that may be used as substrates for target materials. Melting points, and thermal and electrical conductivities are presented for values near 300° K (27° C.). Most values are taken from the CRC Handbook of Chemistry and Physics, 90th ed. [CRC Press, Boca Raton, Fla., 2009]. Other values are taken from various references.
TABLE IVarious Target and Substrate Materials and Selected Properties.AtomicMeltingThermalElectricalMaterialNumberPoint ° C.ConductivityConductivity(Elemental Symbol)Z(1 atm)(W/(m ° C.))(MS/m)Common Target Materials:Chromium (Cr)24190793.7 7.9Iron (Fe)26153880.210.0Cobalt (Co)27149510017.9Copper (Cu)29108540158.0Molybdenum (Mo)42262313818.1Silver (Ag)47 96242961.4Tungsten (W)74342217418.4Potential Substrate Materials with low atomic number:Beryllium (Be)4128720026.6Carbon (C): Diamond6*2300 10−19Carbon (C): Graphite6*1950 0.25(in plane)Boron Nitride (BN)B = 5**20 10−17N = 7Silicon (Si)1414141241.56 × 10−9Silicon CarbideSi = 1427980.4910−9(β-SiC)C = 6Sapphire (Al2O3)Al = 13205332.5 10−20(∥ C-axis)O = 8* Carbon does not melt at 1 atm; it sublimes at ~3600° C.** BN does not melt at 1 atm; it sublimes at ~2973° C.
Other ways to increase the brightness of the x-ray source are: increasing the electron current density, either by increasing the overall current or by focusing the electron beam to a smaller spot using, for example, electron optics; or by increasing the electron energy by increasing the accelerating voltage.
However, these improvements have a limit, in that all can increase the amount of heat generated in the interaction volume. If too much heat is generated within the target, damage can occur. One prior art technology developed to improve thermal management and mitigate this damage is the rotating anode system, illustrated in FIGS. 5A and 5B. In FIG. 5A, a cross-section is shown for a rotating anode x-ray source 580 comprising a target anode 500. The target anode 500 is connected by a shaft 530 to a rotor 520 supported by conducting bearings 524 that connect, through its mount 522, to the lead 22 and the positive terminal of the high voltage supply 10. The rotation of the rotor 520, shaft 530 and anode 500, all within the vacuum chamber 20, is typically driven inductively by stator windings 525 mounted outside the vacuum.
A top view of the target anode 500 is shown in more detail in FIG. 5B. The edge 510 of the rotating target anode 500 may be beveled at an angle, and the emitter 11 of the electron beam 511 directs the electron beam onto the beveled edge 510 of the target anode 500, generating x-rays 888 at an electron beam spot 501. As the electron beam spot 501 generates x-rays, the irradiated spot in the target heats up. However, as the target anode 500 rotates, the heated spot moves away from the beam spot 501, and the electron beam 511 now irradiates a cooler portion of the target anode 500. The hot spot has the time of one rotation to cool before becoming heated again when it again passes through the beam spot 501. By continuously rotating the target anode 500, x-rays appear to be generated from a fixed single spot, while the total area of the target illuminated by the electron beam is substantially larger than the electron beam spot, effectively spreading the electron energy deposition over a larger area (and volume).
Another approach to mitigating heat is to use a target with a thin layer of target x-ray generating material deposited onto a substrate with high heat conduction. Because the interaction volume is thin, for electrons with energies up to 100 keV the target material itself need not be thicker than a few microns, and can be deposited onto a substrate, such as diamond, sapphire or graphite that conducts the heat away quickly. [Diamond mounted anodes for x-ray sources have been described by, for example, K. Upadhya et al. U.S. Pat. No. 4,972,449; B. Spitsyn et. al. U.S. Pat. No. 5,148,462; and M. Fryda et al., U.S. Pat. No. 6,850,598].
The substrate may also comprise channels for a coolant, that remove heat from the substrate [see, for example, Paul E. Larson, U.S. Pat. No. 5,602,899]. Water-cooled anodes are used for a variety of x-ray sources, including rotating anode x-ray sources.
The substrate may in turn be mounted to a heat sink comprising copper or some other material chosen for its thermally conducting properties. The heat sink may also comprise channels for a coolant [see, for example, Edward J. Morton, U.S. Pat. No. 8,094,784]. In some cases, thermoelectric coolers or cryogenic systems have been used to provide further cooling to an x-ray target mounted onto a heat sink.
Although these techniques to mitigate heat in x-ray sources have been developed, there are still limits on the ultimate x-ray brightness that may be achieved, particularly when the source is to be coupled to an x-ray optical system that collects x-rays only in a limited angular range. There is therefore a need for an x-ray source that may be used to achieve higher x-ray brightness through the use of a higher electron current density into a predefined angular range, and is still compact enough to fit in a laboratory or table-top environment.